This invention relates to an apparatus and method for detecting pipette tip obstructions and more particularly to a method and apparatus for detecting a blood clot obstruction at a pipette during aspiration of a blood sample for sample analysis.
In known automatic sampling systems, predetermined test sample volumes of blood or serum react with predetermined volumes of reagent to produce a measurable test result that is the basis for an analytical determination of blood characteristics. Known sampling systems usually include a pipette or sampling probe to aspirate test sample from a container such as a tube or cuvette.
If the pipette aspirates less than the predetermined test sample volume a flawed test result can occur since accurate test results require a predetermined test sample volume to react with a predetermined amount of reagent. As with many analytical processes a limited volumetric difference from the predetermined test sample volume may be tolerable for test purposes.
The volume of aspirated test sample can be predetermined when the aspiration occurs over a known time duration since the tip opening of the pipette is known and the aspiration flow rate of test sample into the pipette is known from the operational characteristics of the aspiration pump. Thus the aspiration of test sample is usually time controlled with the aspiration time corresponding to a predetermined volume of aspirated test sample.
It is also known that a blood sample in a container may contain a clot or clots and that such clots can close or obstruct the probe tip, impeding the aspiration. If movement of fluid into the probe is fully or partially blocked during a fixed aspiration time cycle a reduced volume of test sample will be aspirated. In some instances a clot or obstruction remains in the flow path of fluid directed into the probe and never clogs the probe. Nevertheless a flow path obstruction can also reduce the flow rate of fluid aspirated in a given time, thus preventing the aspirated test sample from reaching the predetermined volume in the preset aspiration time.
In other instances a clot or obstruction will enter and clog the probe, thereby reducing the flow rate of fluid being aspirated and preventing the predetermined volume of fluid from being aspirated in a preset aspiration time. Subsequent aspirations by the clogged probe will also be less than the expected predetermined sample volume. Since a limited volumetric difference from the predetermined test sample volume may be acceptable for test purposes, it is desirable to provide a system and method that distinguishes between acceptable aspiration volumes and unacceptable aspiration volumes.
Previous attempts to detect clot obstructed aspirations are generally based on a detection of pressure conditions during aspiration. For example, U.S. Pat. No. 5,503,036 to Nguyen et al shows an obstruction detection circuit with a pressure sensor to detect abnormal pressure conditions in a sample probe. The abnormal pressure conditions signify the presence of an obstruction.
U.S. Pat. No. 3,754,444 to Ure et al shows a medical sampling device that includes a clot detector that relies on a pressure increase in the fluid system to set off an alarm.
European Publication 0289946A2 to Itoh shows a pressure switch to detect pipette clogging. European Publication 0571100A1 to Tolonen shows a pressure transducer to detect pipette clogging. European Publication 0658769A1 to Takeda et al shows the use of pressure readings to detect a leakage condition in a pipette.
In most known blood clot detection systems the sample volume aspirated is relatively large, such as for example, approximately 200 microliters. Aspiration of a 200 microliter sample usually provides recognizable vacuum measurements in the aspiration line that can be detected by a pressure transducer or other pressure sensing device. A vacuum measurement that is made under normal unobstructed aspiration conditions can be used as a reference standard. Vacuum measurements are then made for each test sample aspiration and any deviation from the reference standard by a predetermined amount can indicate an obstructed aspiration. An appropriate warning signal can be generated in response to a deviant vacuum measurement to alert an operator that an obstructed probe condition has been detected.
Unless otherwise indicated the term "clot detection" as used hereinafter is intended to refer to the detection of an obstruction of an aspiration probe, caused by an internal clot in the probe, an external clot in the aspiration path outside the probe or any other obstruction that is internal or external of the probe that serves to reduce the amount or flow rate of fluid being aspirated in a predetermined time period.
Clot detection problems arise when an aspirated sample volume is relatively small such as for example approximately one to seven microliters. Aspiration of a one to seven microliter sample can be accomplished with a relatively small amount of vacuum in the range of one centimeter of water. However transducer measurement of a vacuum level of one centimeter of water is difficult to recognize because even a sensitive transducer has a noise level that is likely to obscure the detection or measurement of a vacuum level of one centimeter of water.
At vacuum levels in the range of one centimeter of water the transducer noise level also obscures the recognition of deviant vacuum levels that may indicate an obstructed aspiration. Thus far it is not feasible to rely on transducer based vacuum measurement for clot detection when aspirating samples of one to seven microliters.
It is therefore desirable to provide a method and apparatus for clot detection at an aspiration probe when the aspirated volumes are of the order of one to seven microliters and the vacuum levels are in the range of one centimeter of water.